A variety of methods were employed to extract cell wall antigens from B. canis. Their specificity was examined. One antigen (lipopolysaccharide-protein complex) has an unusually high degree of specificity, allowing discrimination of most infected animals from those with heterospecific antibodies. The physical, chemical and serological properties of 3 cell wall antigens extracted by hot saline or sodium desoxycholate were investigated extensively, using two B. canis strains of differeing phenotypes. One was very mucoid and virulent; the other less mucoid and less pathogenic. An antigen designated "2R" has the greatest species specificity for B. canis; two closely related rough brucellae (B. ovis and B. abortus 45/20) did not cross-react with antigen 2R. During the past year we also studied more than 150 human sera from patients in Mexico with undiagnosed febrile illness; approximately 39% agglutinated the slide agglutination test antigen, while immunodiffusion tests revealed 13% seropositive. Approximately 11% of Mexican (D.R.) dogs have been found to have a bacteremia; 25% were judged seropositive after extensive study. Addition of 2-mercaptoethanol to sera prior to use in the slide test improved greatly the test specificity. Bacteriological and serological profiles on a group of dogs infected shortly after birth, and in the 4th year of study, continue.